Attention is paid to the wavelength division multiplexing (WDM) system from the viewpoint of enhanced speed and capacity of optical communication. The optical multiplexer/demultiplexer is indispensable device for the WDM system. The devices in the type used by being coupled with a single-mode fiber are particularly crucial. The reason is that using a single-mode fiber can transmit optical signals at less deterioration of signal waveform.
As a conventional device, there is known an optical demultiplexer described in publication: Applied Physics Letter, Vol. 61, No. 15, pp. 1754–1756, published in 1992, for example.
FIG. 13 is a plan view of a typical example of the device. This device is made up of a single-mode waveguide 10 of one core, a multi-mode waveguide 2, and a single-mode waveguide array 3 of four cores, with all parts being coupled to series in the optical axis direction on a substrate 1. When the device is used as demultiplexer, a single-mode fiber 4 is coupled to the 1-core side and a light is put in to the single-mode waveguide 10. The light excites in multiple modes at incidence to the multi-mode waveguide 2 and branches to four ways due to the interference among the modes, and then is conducted from the single-mode waveguide array 3 to 4-core single-mode fibers 4′.
The above-said conventional device has its single-mode waveguide 10 located between the 1-core single-mode fiber 4 and the multi-mode waveguide 2, so that the light is incident to the center of the multi-mode waveguide 2. In this case, however, if there is misalignment between the single-mode fiber 4 and the device on its 1-core side, a radiative light 14 of a higher-order mode excites in the single-mode waveguide 10. This higher-order mode light 14 interferes with the 0th-order mode light 13. Due to the fluctuation of light distribution during the propagation, even a small misalignment can cause the incident light going into the multi-mode waveguide 2 to deviate greatly in position and direction from the center axis. FIG. 14 shows the relation in this event among the center axis 11 of the single-mode fiber 4, the center axis 12 of the single-mode waveguide 10, and the peak positions 15 of the light intensity. Other portions of this figure are referred to by the same symbols as those of FIG. 13. There arises a significant inequality in light output among the channels. Therefore, high positioning accuracy is required in setting up the device, and it is difficult to lower the setup cost based on a simple passive alignment method.
In view of the foregoing situation, it is an object of the present invention to provide an optical multiplexer/demultiplexer which has large tolerance of setup positioning error against the single-mode fiber and allows modular setup based on a low cost simple passive alignment method.
Japanese Patent Laid-Open No.H10 (1998)-48458 describes an example directly coupling of a multi-mode fiber to a multi-mode waveguide. However, this patent publication pertains solely to a technique of the use of a multi-mode fiber.